sched, bpf: make skb->priority writable

{cls,act}_bpf can now set the skb->priority from an eBPF program based
on various critera, so that for example classful qdiscs like multiq can
update the skb's priority during enqueue time and further push it down
into subsequent qdiscs.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -1721,6 +1721,7 @@ static bool tc_cls_act_is_valid_access(int off, int size,
 		switch (off) {
 		case offsetof(struct __sk_buff, mark):
 		case offsetof(struct __sk_buff, tc_index):
+		case offsetof(struct __sk_buff, priority):
 		case offsetof(struct __sk_buff, cb[0]) ...
 			offsetof(struct __sk_buff, cb[4]):
 			break;
@@ -1762,8 +1763,12 @@ static u32 bpf_net_convert_ctx_access(enum bpf_access_type type, int dst_reg,
 	case offsetof(struct __sk_buff, priority):
 		BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, priority) != 4);
 
-		*insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg,
-				      offsetof(struct sk_buff, priority));
+		if (type == BPF_WRITE)
+			*insn++ = BPF_STX_MEM(BPF_W, dst_reg, src_reg,
+					      offsetof(struct sk_buff, priority));
+		else
+			*insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg,
+					      offsetof(struct sk_buff, priority));
 		break;
 
 	case offsetof(struct __sk_buff, ingress_ifindex):